1. Field of the Invention
The present invention relates to a stent for placement in a body tube. The stent includes a flexible support structure of wires, wherein the wires are of a material having a shape memory function.
The present invention also relates to a method for changing the configuration of a stent of the above-described type which has been placed in a body tube.
2. Description of the Related Art
Stents are hollow cylindrical spacer members which are implanted operatively, percutaneously or endoscopically in order to keep tubes in the body open, for example, the trachea, bronchial tubes, esophagus, biliary duct, urinary passage, blood vessels and the like.
Such stents must have a restoring force for withstanding an external compression which is caused, for example, by a tumor or a lymph node, or a vessel stricture resulting from cauterization, sclerosis or cicatrization.
Stents of different configurations and construction are known for keeping open a stenosis. The stents may be of plastic construction, metal construction or hybrid construction. Many stents have fixed end diameters and are self-expanding, as disclosed in DE-GM 91 16 881 or DE-OS 42 40 177.
In other embodiments, the diameter of the stent can be changed and adapted to the respective anatomical situation by suitable means, for example, balloons or spreaders. Such a stent is known from U.S. Pat. No. 5,201,901.
Also known in the art are stents of a so called shape memory alloy, for example, from DE-OS 42 19 949 and. For example, nitinol is a shape memory alloy. Nitinol has two distinct states which occur depending on the temperature. After a pretreatment, nitinol is martensitic, i.e., plastically deformable, in the cold state and does not have a relevant elastic restoring force. After heating, nitinol changes into an austenitic, elastic state.
The shape memory property is utilized for the self-expansion of various stents.
The restoring force, i.e., the force with which the stent counteracts a compression, depends on the construction and the thickness of the wires. There are also stents of thermoplastic synthetic material, such as polyurethane, in which the restoring force depends on the material thickness.
Depending on their construction and the type of application, the known stents operate satisfactorily. However, it is desirable to be able to change the restoring force of a stent in vivo, i.e., after placement in the body, in order to be able to adapt to different conditions, as they occur, for example, as a result of the growth of a tumor.